Singlet Fission in 9,10-Bis(Phenylethynyl)Anthracene Thin Films Youn Jue Bae, Gyeongwon Kang, Christos D

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Cite This: J. Am. Chem. Soc. 2018, 140, 15140−15144 pubs.acs.org/JACS

Singlet Fission in 9,10-Bis(phenylethynyl)anthracene Thin Films Youn Jue Bae, Gyeongwon Kang, Christos D. Malliakas, Jordan N. Nelson, Jiawang Zhou, Ryan M. Young, Yi-Lin Wu, Richard P. Van Duyne, George C. Schatz, and Michael R. Wasielewski* Department of Chemistry and Institute for Sustainability and Energy at Northwestern, Northwestern University, Evanston, Illinois 60208-3113, United States

*S Supporting Information

9,10-Bis(phenylethynyl)anthracene (BPEA) is a robust, ABSTRACT: Singlet fission (SF) in two or more industrial dye that has been widely used for chemiluminescent − electronically coupled organic chromophores converts a lighting,14 16 photoswitches,17 optical waveguides18 and solar high-energy singlet exciton into two low-energy triplet cells.19 Its good chemical and thermal stabilities along with its excitons, which can be used to increase solar cell facile synthesis and functionalization allow a wide range of efficiency. Many known SF chromophores are unsuitable different photochemical and photophysical applications.20 for device applications due to chemical instability and low BPEA is a strong blue light absorber with a near unity triplet state energies. The results described here show that quantum yield (Φ = 1) of green fluorescence.21 Various ffi fi f e cient SF occurs in polycrystalline thin lms of 9,10- structural modifications of BPEA have been reported and bis(phenylethynyl)anthracene (BPEA), a commercial dye highlight its tunable electronic properties.15,22 Although BPEA that has singlet and triplet energies of 2.40 and 1.11 eV, has been suggested as a potential SF candidate,2 only one study respectively, in the solid state. BPEA crystallizes into two of SF in BPEA nanoaggregates has appeared following polymorphs with space groups C2/c and Pbcn, which submission of this manuscript.23 Moreover, a recent report undergo SF with k = (109 ± 4 ps)−1 and k = (490 ± − SFA SFB on 9,10-bis(TIPS)anthracene aggregates in solution showed 10 ps) 1, respectively. The high triplet energy and efficient 24 ± only modest SF yields. Crystalline BPEA powder obtained SF evidenced from the 180 20% triplet yield make from Sigma-Aldrich contains 59.90 ± 0.02% C2/c and 40.10 ± BPEA a promising candidate for enhancing solar cell 0.02% Pbcn polymorphs (Figure 1a) based on a quantitative performance. powder X-ray diffraction (PXRD) analysis (Figure S1).17 BPEA films were prepared by thermal vapor deposition under high vacuum onto a sapphire substrate followed by two inglet fission (SF), the process by which a singlet exciton annealing methods: solvent vapor annealing with dichloro- S created in an assembly of two or more interacting methane for 12 h or thermal annealing at 120 °C for 3 h. chromophores energetically down-converts into two triplet Samples for PXRD analysis were scraped off the annealed films excitons, was first discovered in 1965 in crystalline to remove preferred orientations. Analysis of the PXRD data anthracene.1 Over the past decade, SF chromophores have (Figure 1b) using full-profile Rietveld fitting25 against the two 17,26 attracted renewed attention because of their ability to reported crystal structures of BPEA reveals that the overcome the Shockley−Quiesser limit on the theoretical solvent-annealed film contains 61.2 ± 0.5% of the C2/c and

Downloaded via NORTHWESTERN UNIV on November 26, 2018 at 16:04:51 (UTC). ± efficiency of single-junction solar cells.2,3 Specifically, SF 38.8 0.5% of the Pbcn polymorphs, while the thermally fi chromophores can mitigate thermalization losses, where the annealed lm contains >97% of the Pbcn polymorph.

See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. Φ BPEA in solution has a 2.64 eV S1 state energy, f = 1, and a absorbed photon energy in excess of the band gap is lost to ± 27 4 3.17 0.02 ns S1 lifetime (Figure S2). In both the solvent- heat. The triplet excitons can be harvested by either energy fi and thermally annealed lms, the S1 energy decreases to 2.40 transfer or charge transfer to ultimately create charge − 5,6 7 eV (Figure 2), which is expected from transition dipole dipole carriers. For example, Dexter-type energy transfer from coupling and orbital overlap-driven electronic coupling among triplet excitons of pentacene and tetracene to lead-based chromophores.28 The fluorescence of both the solvent and quantum dots has been demonstrated.8,9 Charge transfer from fi fi Φ thermally annealed lms is signi cantly quenched to f = 0.04 the triplet exciton is also possible if the SF chromophores are ± 0.01 and 0.27 ± 0.04, respectively. BPEA phosphorescence paired with a suitable electron acceptor or donor, following the in glassy 2-methyltetrahydrofuran at 77 K is essentially 10,11 architecture of conventional organic solar cells. According undetectable because of its extremely low spin−orbit-induced to detailed balance, the maximum theoretical efficiency for a intersystem crossing quantum yield (10−5).20 Even in thin films single-junction photovoltaic cell is achieved with a band gap of where a significant triplet population is formed (vide infra), 1.1−1.3 eV.12 Thus, the triplet energy of a SF chromophore phosphorescence is undetectable at 77 K. Thus, we used incorporated into a photovoltaic device should be >1.1 eV. In palladium octabutoxyphthalocyanine, PdPc(OBu)8, which has addition, because Frenkel excitons in organic chromophores have large binding energies, higher triplet energies facilitate Received: July 16, 2018 charge transfer to a nearby donor or acceptor.13 Published: October 29, 2018

(Figure S7) gives the lower limit and based on these data, ± fi E(T1) = 1.11 0.13 eV, so that SF in the BPEA lm should be nearly isoergic, and somewhat slower than for exoergic polyacenes.31,32 However, efficient SF is observed even in − films of endoergic chromophores.33 35 Figure 3 shows the femtosecond transient absorption (fsTA) spectra of the solvent-annealed 170 nm BPEA film following

Figure 1. (a) BPEA dimer from the crystal structure of the C2/c (left) and Pbcn (right) polymorphs. (b) Comparison of the simulated PXRD patterns of the C2/c (red) and Pbcn (blue) polymorphs with the PXRD pattern of the solvent-annealed (black) and the thermally annealed (green) BPEA thin ﬁlms.

Figure 3. (a) FsTA spectra of a solvent-annealed 170 nm BPEA film using (a) 414 nm, 100 fs, 1 kHz laser pulses, (b) species-associated spectra and rate constants and (c) comparison of the transient difference spectrum of the triplet state to the thermal difference spectrum. Wavelengths for panels b and c are shown in reciprocal Figure 2. Steady-state absorbance (black) and emission (red) spectra space to highlight the blue region of the spectrum. Reported values of (a) solvent-annealed (b) thermally annealed polycrystalline BPEA are the average and std. deviation from multiple experiments. thin films. excitation with a 414 nm, 100 fs laser pulse at a 1 kHz a 1.24 eV triplet state energy, as a photosensitizer to produce repetition rate and an excitation density of 7.8 × 1019 cm−3. the BPEA triplet state in a film.29,30 Following selective The transient spectrum (Figure 3a) at early times exhibits photoexcitation of PdPc(OBu)8 at 720 nm (Figure S3), rapid ground-state bleaching (GSB) overlapped with stimulated 3* intersystem crossing populates PdPc(OBu)8, which then emission (SE) near 420 and 550 nm, which is consistent with energy transfers to BPEA to produce 3*BPEA (Figure S4). the steady-state absorption and emission spectra (Figure 2), 3* ← This observation places an upper bound on the BPEA energy and Sn S1 excited-state absorption (ESA) features from 550 ≤ fi as E(T1) 1.24 eV in the solid lm. In addition, the to 1400 nm, which are broadened compared to those in observation of delayed fluorescence (Figure S6) from the solution (Figure S2b). At later times, the ESA at 445−495 nm fi 3* ← photoexcited solvent-annealed lm indicates that the BPEA is assigned to Tn T1 absorption based on the triplet- energy must be close to 1.20 eV, which is half the S1 energy, sensitized spectrum (Figures S4 and S5) with an overlapping 2.40 eV. Observation of singlet oxygen emission at 0.98 eV GSB feature at 495−550 nm. The kinetic fits and species

15141 DOI: 10.1021/jacs.8b07498 J. Am. Chem. Soc. 2018, 140, 15140−15144 Journal of the American Chemical Society Communication populations are shown in Figure S8. In order to distinguish the transient absorption spectrum of the solvent-annealed BPEA triplet spectrum from the thermally induced spectral shift in film. For the solvent-annealed BPEA film, 100% of the the ground state bleach,36 we compared the species-associated expected bleach was needed to remove the negative transient triplet spectrum to the ground state thermal difference spectra features at 52 ns (Figure 4). By correcting for the spectrum between 50 and 25 °C(Figure 3c). Although the triplet decay experienced by 52 ns, the time-independent similar spectral shape indicates that there is some degree of triplet yield extrapolated to 1.5 ns, when SF is essentially thermal effect present, the magnitude is smaller than the complete, is 180 ± 20% (see SI). A similar procedure applied ← Δ fi ± observed Tn T1 spectrum and the overall A of the thermal to data for the thermally annealed BPEA lm gives an 80 spectrum is negative. In addition, singlet oxygen emission 20% triplet yield (Figure S17). Application of the spectral (Figure S7) supports formation of triplet in the thin film and deconvolution method gives similar values for the triplet yield the polymorph-dependent triplet yield suggests that ΔA at (Figure S18). ff later pump−probe delays results from the triplet. Similar In order to understand the origin of the SF rate di erence spectra are obtained for the thermally annealed 310 nm BPEA between the two polymorphs, we compared the detailed film, except that the SE feature is dominant (Figure S9a). Since structures of the dimer units in the crystal structure and ← calculated their effective electronic couplings.31,35,41 Among the initial rapid decay of the GSB and Sn S1 ESA imply that − the three different neighboring dimers of each polymorph singlet singlet annihilation (SSA) occurs at the excitation − density used,31,32 the data were globally fit using a kinetic (Figures S19 S21), Figure 1a shows the dimer that has the model that includes SSA and the relative polymorph largest electronic coupling (Tables S3 and S4). Comparing the ± −1 packing between the two dimer units from each polymorph, populations (see SI) to yield kSFA = (90 30 ps) and kSFB ± −1 both have the same 0.80 Å longitudinal slip distance, and = (480 40 ps) for the two BPEA polymorphs. Similar π−π values are obtained using fsTA and picosecond time-resolved similar distances, 3.40 Å for C2/c and 3.45 Å for Pbcn. The largest structural difference between the dimer units fluorescence (TRF) systems having 100 kHz repetition rates 17 16 −3 occurs in the lateral slip distances, which are 4.06 Å for C2/c with excitation densities of 10 and 10 cm , respectively. ff − and 3.33 Å for Pbcn. Such di erences change orbital overlap, (Figures S10 S13). TRF spectroscopy was also used to obtain ff ff − ± resulting in di erent e ective electronic coupling between the the triplet triplet annihilation rate constant kT = (196 5 ns/ Δ −1 fl two polymorphs. A) responsible for the delayed uorescence (Figure S6b). Considering the first-order coupling of CT states to the As shown in the PXRD pattern (Figure 1), the thermally 1 fi fi initially excited singlet state, (S0S1), and the nal correlated annealed lm is more than 97% Pbcn polymorph, which triplet pair state, 1(T T ), the effective electronic coupling for permits a straightforward assignment of the two observed SF 1 1 the superexchange mechanism, JSE,eff, was calculated using eq rates. Picosecond TRF spectra of the thermally annealed film 42 fi ± 1: (Figures S14 and S15) were globally t to yield kSFB = (490 − 10 ps) 1. Thus, the slower SF rate results from the Pbcn 1 ̂ 1 JVSE,eff = TT11 polymorph, while the faster SF rate derives from the C2/c − polymorph. 0 ̂ 0 2(VVLL LH V HH V HL ) =− TT 1 1 Nanosecond transient absorption (nsTA) spectra (Figure [−]+[−]EE(CT) (TT) EE (CT) (S1 ) S16) of the BPEA films were employed to determine their (1) triplet yields using the singlet depletion method,37 which has The first term on the right-hand side of the equation will be been demonstrated to be effective in obtaining SF yields 34,38−40 ignored because the direct two-electron coupling is small (Figures 4, S15, Table S1). The number of excited 43 compared to the four one-electron couplings. VLL and VHH molecules was calculated from the excitation pulse energy, laser are the one-electron couplings of the LUMO and HOMO of spot size, and film thickness to obtain the expected ground- the two molecules, respectively, whereas VLH and VHL are one- state bleach spectrum; then, the number of triplet states was electron couplings between the LUMO of the first molecule estimated by adding the scaled ground-state bleach to the and the HOMO of the second molecule, and vice versa. Using the values specified in Tables S3 and S5 (see the SI for computational details), JSE,eff is 3.73 meV for Pbcn and 7.80 meV for C2/c. Assuming the reorganization energies are similar for both polymorphs, and since the SF rate is | |2 proportional to J SE,eff , the SF rate for C2/c is estimated to be 4.4 times faster than that of Pbcn. The large JSE,eff value for the C2/c polymorph is mainly due to the significant LUMO− LUMO coupling, VLL, which is 120 meV for C2/c and 11.7 meV for Pbcn (Table S4). This large LUMO−LUMO coupling is also visualized in the molecular orbital diagrams shown in Figure S22, where the electron density of the LUMO is mainly localized on the anthracene. The spatial overlap of the LUMOs of the C2/c dimer is larger than that of the Pbcn dimer, resulting in energy splitting of the degenerate LUMOs. − It is widely known that the slip distance is an important Figure 4. Triplet triplet absorption decay of the solvent-annealed 43−46 fi τ ± τ μ factor in determining SF rates. In order to understand the BPEA lm ( 1 =80 10 ns (94%); 2 >4 s (6%) following 416 nm, 7 ns pulses at 10 Hz. Inset: the triplet−triplet absorption spectrum effect of lateral slip distance (ΔX), the effective electronic determined from the nsTA spectrum at 52 ns using the singlet coupling and the one-electron couplings were calculated with depletion method. the fixed π−π and longitudinal slip distances of the C2/c

15142 DOI: 10.1021/jacs.8b07498 J. Am. Chem. Soc. 2018, 140, 15140−15144 Journal of the American Chemical Society Communication

Δ | |2 polymorph, where X is varied (Figure 5). The value of JSE,eff ■ AUTHOR INFORMATION Δ for the C2/c polymorph decreases as X increases from 3.0 Corresponding Author *[email protected] ORCID Gyeongwon Kang: 0000-0002-8219-2717 Christos D. Malliakas: 0000-0003-4416-638X Jiawang Zhou: 0000-0002-2399-0030 Ryan M. Young: 0000-0002-5108-0261 Yi-Lin Wu: 0000-0003-0253-1625 Richard P. Van Duyne: 0000-0001-8861-2228 George C. Schatz: 0000-0001-5837-4740 Michael R. Wasielewski: 0000-0003-2920-5440 Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award DE-FG02-99ER14999 (M.R.W., experiments) and by the National Science Foundation under grant no. CHE-1760537 (G.C.S., theory). G.K., R.P.V.D., and G.C.S. acknowledge support from the Air Force Office of Scientific Research MURI (FA9550-14-1-0003, theory). 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15144 DOI: 10.1021/jacs.8b07498 J. Am. Chem. Soc. 2018, 140, 15140−15144